darkoshi | Entries tagged with eclipse

I know it's not true, but it seems like *everybody* got to see totality except me (and of course my mom and Qiao, who were with me.)

I know I'm lucky to have been able to experience as much of it as I did experience. To not have even had to travel anywhere to experience it. There are surely people who traveled far from home to see it, and had clouds get in the way; that would have been even worse bad luck. And I'm sure there were many people who were in the path, but were unable to watch it for other reasons.

What with the weather forecasts, it's not like I was really *expecting* to be able to see it. I knew it was a long-shot. It seemed, though, that even if I were to drive somewhere else with a better forecast, it would be just as much a matter of luck, along with the risk of being stuck in traffic during the actual event.

But that break in the clouds here as it was getting closer to totality really made it seem like we were going to get to see it after all... and then... that big dark cloud, sob.

I'm glad that on Monday evening, there was one announcer on TV, who when the others were talking about it, said "well actually I didn't get to see it; in my area it was raining". It made me feel less alone in my bad luck. He's one of only two people I've heard on TV who were in the path of totality, who said they didn't get to see it due to clouds or rain.

I wonder, if I had seen it, if it would have seemed really special, or just a neat glowing disk in the sky. I suspect that latter, because nothing feels really special to me. So it's not like I probably even missed a great experience.

Oh, man. There's a pretty good video of it on this page. But apparently, totality was even visible in Charleston and on the coast, though the weather map had been showing the worst cloud cover there. ::sob, I'm cursed::

... oh, wait. Here's some other people who had clouds. See there, self, you're not the only one with disappointment.

..

I saw the crescent moon this evening, around 9pm. Sunset was around 8pm.

It was partly cloudy here today.

I got to see a lot of the partial eclipse. A few minutes before totality, a big cloud moved in front of the sun, so I didn't get to watch that most special part. But I did get to experience the sky darkening (not nearly as dark as in the middle of the night; more like shortly after sunset), and the temperature dropping, and the wind whipping up, and a few cicadas starting to buzz, and part of the horizon looking pink.

It was very similar to a big thunderstorm building up. That's what the dogs seemed to think, anyway, as they rushed for the porch and pawed at the front door to be let inside. I let them in and then us humans remained outside to watch.

One curious thing is that the partial eclipse started with the moon entering the upper right portion of the sun, and ended with the moon leaving the upper left portion of the sun (when viewing generally southwards for both). That's not what I had expected. Maybe I'm mistaken about what direction I was looking when it started.

Many people here in town did get to see the totality; it just depended on where one was, and where the clouds were. But in some areas, it even rained.

My neighbor was visiting a relative at the hospital this afternoon, and she told me that a lot of the hospital staff went outside to view the eclipse (but staff who were needed inside stayed in). She said that the Emergency Room remained open, but the normal operating rooms were closed for that time period. That answers one of the things I had wondered about.

Click to enlarge...

Follow-up to this post.

In the prior post, I pondered about angles. In particular, about the ~13 degrees per day that the moon orbits around the earth, and whether that angle would still look like 13 degrees to me, when measured from the surface of the earth.

The angle would *not* be exactly the same. However, because of how far away the moon is compared to the size of the earth, the difference in angle is very small. That difference can be calculated using trigonometry.

Here's a new diagram. All figures mentioned below are approximations or averages.

The angle measured from the center of the earth is 13 degrees.
"X" is the corresponding angle measured from the surface, which will be calculated.
"D" is the distance from the center of the earth to the moon: 384400 km
"R" is the radius of the earth : 6371 km

I've drawn 2 right triangles such that both have the same "opposite" side, with length "O".
The length of the adjacent side for the X-angled triangle is "A".
The length of the hypotenuse for the X-angled triangle is "B".
The length of the adjacent side for the 13-degree-angled triangle is A + R.
The length of the hypotenuse for the 13-degree angled triangle is D.

sin 13 degrees = O / D
O = D * sin 13 = 384400km * 0.2249511 = 86471 km

cos 13 degrees = (A + R) / D
A = (cos 13) * D - R = (0.9743701 * 384400) - 6371 = 368176.85

tan X = O / A
X = arctan(O / A) = arctan (86471 / 368176.85) = 13.217

So, the corresponding angle from the surface of the earth is ~13.2 degrees.

For the last 3 days, I've seen the crescent moon in the sky during the late morning.

2017/08/16, 10:16am EDT:

2017/08/17, 8:43am EDT:

(On 2017/08/18, I saw the moon around 10:30am, but didn't think of taking a photo.)

But I have been unable to find the moon in the sky around 2:30pm (during my lunch breaks). I've been wondering why I can't find it in the afternoon.

(No wonder I've never paid much attention to the path of the moon in the sky. At night, I'm usually inside or asleep. In the daytime, even when the moon is in the sky, it's hard to see.)

On all 3 days, it's been partly cloudy, with today being the least cloudy. So it's possible the moon was behind a cloud. But as much as I've searched the sky, it seems unlikely it's *always* been behind a cloud.

As of today (2017/08/18) at my location, per the NOAA solar calculator (Find Sunrise, Sunset, Solar Noon and Solar Position for Any Place on Earth), solar noon is around 1:30pm. So at 2:30pm, the sun is still fairly high overhead.

On 8/16, it was 5 days before new moon and the eclipse, so the moon would have been about 5 * 13 = 65 degrees away from the sun. So that was most likely too near the horizon for me to see, as there are some trees and buildings around.

On 8/17, the moon would have been 4 * 13 = 52 degrees away from the sun. I think I should have been able to see it at that angle.

Today on 8/18, the moon would have been 3 * 13 = 39 degrees away from the sun. Surely I should have been able to see it at that angle.

The closer we get to the new moon, the thinner the crescent is. So the harder it is to see. It is hard to find a tiny arc of white in a light blue sky, and even more so when there are distracting white clouds around. But is that the only reason I haven't found it?

Per this page: Finding the Moon, crescent moons are "not observable" except right before sunset or after dawn. But I've seen it at 10:30am which isn't right after dawn. So I think it would be more accurate to say "not easily observable".

If I can see it at 10:30am when the sun is already bright in the sky, why shouldn't I be able to see it at 2:30pm?

I got to wondering whether how I think of the angles in the sky is wrong. I am thinking of 45 degrees as being the distance from straight overhead to a point halfway to the horizon. But the 13 degrees that the moon moves per day is in relation to the center of the earth, not to my spot on the surface of the earth. Therefore, is how I'm visualizing the angles in the sky wrong?

When the moon orbits 45 degrees around the earth, is that a much greater distance than the distance I see from overhead to halfway to the horizon?

But... as can be seen in the diagram, the larger you draw the earth, the closer the 45 degrees gets to one's visible horizon~~, and it would eventually even pass below the horizon~~. Yet I've been able to see the moon in the mornings, and the distance between it and the sun hasn't seemed such a large angle. So surely the above diagram can't be right.

(Update #2, 2017/08/20: I've figured it out. The diagram is basically correct, but my assumption about the 45 degree line eventually passing below the horizon was wrong (just because I don't draw the horizon line to infinity, doesn't mean it doesn't go to infinity). If the angle to the moon as measured from the center of the earth is 45 degrees (from directly overhead), then the angle as measured from the surface of the earth would be more than 45 degrees. But because the distance to the moon is so large in comparison to the size of the earth, the angle is only slightly more. See follow-up post.)

On the same topic, I got to wondering how much of the sky / celestial sphere am I actually capable of observing from a point on the earth, at any moment in time. Ie. if I turn all the way around, looking towards the horizon, and up above me, how much of the sphere of the sky which surrounds the earth, am I seeing?

Based on the diagram, the amount of sky seen would not be half the sphere, as I've previously assumed. Yet again, the larger one draws the earth, the less of the sky one would seem to see. Surely that can't be right?

Based on these answers, it sounds like you should be able to see half of the sky at any time. But I don't understand the formulas and calculations listed.

Update (afternoon of 2017/08/19):

Today, the morning of 8/19, around 7:40am and again at 10:20am, I wasn't able to find the moon in the sky, even though it was clear with no clouds. So as of 2 days before new moon, the crescent must be too small and faint to see in the daytime. Perhaps a clear sky being so much brighter than a partially cloudy sky, also makes it harder to see.

MoonCalc.org - shows you the current position of the moon in the sky, and moonrise/moonset directions, for any position you select on the map.

Sun Locator Lite - a free app which lets you find the sun and moon by pointing the phone at the sky (as long as the phone has an internal compass/magnetometer - mine doesn't, but Qiao's does). The Pro version lets you get information for any day and time of the year.

Today, 2 days before the eclipse, the moon should be about 2 * 13 = 26 degrees from the sun. I used the above Sun Locator app to find the position of the moon and compare it to the sun's position, and estimated the angle between them. If anything, it seemed less than 26 degrees, not more. So that indicates that there's something wrong with my thinking in terms of the above diagram. But where have I gone wrong? I still haven't figured that out.
(And even with the app to show me its exact location, I still can't see the crescent moon in the afternoon sky.)

But I did have an epiphany on how much of the sky is visible from a point on earth at a single moment in time. It depends on what I'm calling the "sky". I think of the sky as a sphere centered around the earth, upon which I see moon, sun, stars, clouds, etc. But there are many such possible spheres around the earth, different distances from the center of the earth.

How much of the sky is seen depends on which of those spheres one considers. If one considers a sphere which is say, 10 kilometers above sea level, you can calculate the surface area of that sphere. The earth's diameter is 12,742 km. So the sphere's diameter would be 12,752 km, its radius (r) would be 6376 km, and it's surface area would be 4*pi*r^2.

[ another interesting thought... For an infinitely thin sphere, the size of the inside and outside surface areas should be the same, right? But how can that be? I can't visualize them being the same size. ]

Imagine that we cut a small slice, 10 km deep, from the top of that sphere. We can then calculate the surface area of that slice (with some formula, which I would have to look up.) That would tell us how much of the whole sphere we can see at a single moment, and it would be a fairly small portion.

But now, consider a sky-sphere with a much larger radius of 5 light-years - reaching the nearest stars - or even larger. At such distances, the diameter of the earth is minute in comparison - it can be considered negligible. A plane which touches the surface of the earth at one point is practically the same as another parallel plane which intersects both the center of the earth and the sphere. Either way, half of the sphere is above the plane, and half below. So the person can see half of that sky-sphere.

Now, what about a sphere with radius of 150 million km (about the distance from the earth to the sun)? In comparison to that distance, the earth's diameter is roughly 0.01%.* So again, it's basically negligible, and we can see practically half of the sphere at any moment in time.

..

Other interesting tidbits:

How far away is the horizon? Short answer: About 4 to 5 kilometers away, at standing eye-level for an average-height adult.

I see the moon: introducing our nearest neighbour - has several good diagrams/images.
Per this page, the moon's orbital plane is tilted 5 degrees from the ecliptic. That's not as much as I imagined. But when you add in the 23.5 angle of the earth's axis, the moon can orbit up to 29 degrees above or below the earth's equator.

Lunar Orbital Libration
Libration definition: "a real or apparent oscillatory motion, especially of the moon."

Altitude and Azimuth

* A lot of these numbers are rough calculations I've done, and they may have errors. Please don't rely on any numbers I've posted, without verifying them. If you find an error, please let me know so that I can correct it.

I thought of an easier way to explain why the eclipse shadow travels west to east, even though the moon travels east to west through the sky.

First, here's the general picture from the perspective of the sun, when looking down at the solar plane from above:
The earth revolves around the sun in a counter-clockwise direction, completing a full circuit about every 365 days.
The moon revolves around the earth in a counter-clockwise direction, completing a full circuit about every 28 days.
The earth rotates around its own axis in a counter-clockwise direction, completing a full turn every 24 hours.

Here's the general picture from the perspective of a spot on the earth at the equator, when looking up at the sky:
The sun revolves around the earth in an east to west direction, completing a full circuit every 24 hours.
The moon revolves around the earth in an east to west direction, completing a full circuit about every 24.5 hours (I hope I calcuated that right)

Now to explain why the solar eclipse shadow goes west to east:

Imagine you are standing on the north side of an east-west street, facing south.
The moon is a person walking on that street from east to west.
The sun is another person walking on that street from east to west, except that they are walking slightly faster than the moon, and emitting a bright light.

When the sun is still a fair bit behind the moon, the shadow that is cast from the moon due to the sun's light will point towards the west.
As the sun starts overtaking the moon, walking behind the moon compared to the observer, the shadow that is cast points towards west-northwest.
As the sun continues passing behind the moon, that shadow changes direction, towards to the northwest, then north, then northeast, then east-northeast.
So even though both the sun and moon are going east-to-west, the shadow goes west-to-east.

Maybe that is totally obvious to other people? I mean, it seems pretty obvious to myself now that I've explained it.

.

It's actually more complicated than that, of course.

The sun's path does go from east to west rather consistently, even though during the summer, the path is higher in the sky (northeast -> northwest) than during winter (southeast -> southwest).

But the moon's path is more dynamic, as it doesn't revolve in the plane of the equator. It may rise in the southeast and set in the northwest. Or it may rise in the northeast and set in the southwest. (right? I haven't ever really paid much attention to the moon's path, but I must have learned that somewhere.)

Because of that, based on the images I've seen, instead of the moon crossing the sun from right to left, during this eclipse, it will cross it from lower right towards the upper left.

So in the above example, the moon would be on a different street, at an angle to the other street, and the streets would happen to cross each other right at the point where the sun was walking behind the moon.
(although what angles the streets need to be at, and which direction the moon is going on its own street is a bit difficult for me to visualize right now.)
..

I guess it's time to create a new eclipse tag for all these entries, and to rename the eclipse tag I used on a single other post in reference to the software called "Eclipse".

The local news mentioned that the downtown population could double to a million people, from all the people coming here for the total eclipse. And that cell phone service may be affected, due to bandwidth problems from all the extra people. That's something that wouldn't have occurred to me. They advise people to text instead of calling, to save bandwidth.

.

One thing I'm curious about is whether during totality, it will be dark like during the middle of night, or only somewhat dark like when the sun has just dipped below the horizon at dusk, or if it won't even be as dark as that.

I could look it up. But that would be like watching spoilers :-)

I wonder if the street lights will come on.

I wonder how many people will be driving vehicles during totality. I wonder how many people have to work and won't even have an opportunity to go outside to look. I wonder if store employees will have to stay inside. I wonder if people are going to be shopping instead of watching it. I wonder if employees and even managers are simply going to abandon their posts for a few minutes, in order to experience this once in a lifetime event. I wonder if surgeries and doctor's appointments have been scheduled during totality. I wonder if some people just don't care about it. I wonder if some people think it's too hot to go outside just to look at the sky.

I wonder if it will be raining... :-(
As of now the forecast indicates a clear morning, then partly cloudy til 2pm, then a 50% chance of thunderstorms. Totality here is at 2:42pm.

.

This morning I looked in the sky and found the moon in the general area I expected it to be. This afternoon during my lunch break, I looked and wasn't able to find it again. I wonder if the sky is just so bright during midday that a crescent moon is very faint and hard to see. Or whether it was hiding behind one of the clouds.

.

Aaaannnnd I just noticed that I wrote "left to right" a couple times in my prior post, when I meant "right to left". As if it wasn't confusing enough without me flubbing the words too. I fixed it now.

.

Hah. I found another reason it's good I decided to work from home on Monday. At my home, totality will last 20 seconds longer than it will at my work, because my workplace is further from the center line.

This morning, I started wondering why the eclipse will be seen on the west coast before it is seen on the east coast.

I know that the earth spins counter-clockwise (towards the east) when looking at it from above the north pole. And that the moon travels around the earth in the same counter-clockwise direction. And that the earth spins relatively faster. It does a complete rotation (360 degrees) in 24 hours, whereas the moon only travels 1/28th of the way around the earth (360 / 28 ~= 13 degrees) during that time.

So how can the moon's shadow travel from west to east? Isn't the earth spinning into the shadow and out of it in a clockwise west to east direction (the same as it always spins), and so the shadow should appear to move from east to west, just like the moon appears to do in the sky?

According to the answer on PhysLink.com, it has something to do with the moon's orbital velocity being greater than the earth's. But according to this orbital velocity formula, it seems that anything closer to the earth (ie. the earth's surface) would have a greater orbital velocity than something further away (ie. the moon). So that answer seems to be wrong or badly worded, maybe. Of course, if we simply consider velocity, the moon does travel a further distance through space than the earth's surface does, in the same amount of time. But what does that have to do with the eclipse? If it were a race, the earth would still win, rotating faster than the moon revolves.

Here's another page (cached, as the original eclipse2017.org page isn't responding - the website must be swamped) that tries to answer the question. Some of the commenters on that page seem to have the same confusion as I do.

Here's another page (Washington Post) that tries to explain it. Again talking about the speed of the moon compared to the earth.

Watching the various NASA visualizations didn't explain it well to me, because of how the videos keep shifting perspectives. Even in this animation, it looks like the sun must be moving from behind the viewer, to make the shadow move like that.

I think I may finally sort of understand it, but my explanation doesn't match any of the answers I read. So it's probably wrong. But... as the moon moves across the sun from right to left (as seen from the earth while facing south), it's shadow as seen from the earth changes direction. First it points towards the west, then straight, then towards the east.
And while the moon itself, from the earth's perspective, doesn't move far in the sky (and due to the earth's rotation, even appears to be going to the west*), it's shadow moves much faster... that must be why the answers keep mentioning the speed of the moon.. they must be trying to say that the speed of the moon's shadow across the face of the earth matches the moon's speed in space. I suppose that is logical, even though it isn't very intuitive to me**.

* But the sun appears to move to the west faster than the moon, so the moon does still cross the sun from right to left, even though they are both moving to the west.

So the moon's shadow moves quickly from the west to the east.

Right? Maybe? Sort of?

** Because the moon doesn't move in a straight line, but rather circles the earth. And those x-thousand miles per hour it moves up in space only correspond to y-hundred miles down on the earth... Oh jeez, now I'll start doubting my above explanation again...

Ok, thinking about it more. The shadow moves west to east like I explained above, because the moon crosses the sun from west to east. The speed of the moon through space around the earth affects the speed of the shadow, but it's not a direct x = y equation. The faster the moon moves across the face of the sun, the faster the shadow sweeps across the land from west to east. Since the shadow is sweeping through an arc (sort of), the far end of the shadow will pass a different distance during that time, depending from how far away you measure it... which for us is based on the distance between the moon and earth. So the speed of the shadow depends on that distance, and on the speed of the moon's revolution, and on the speed of the earth's rotation, and the size of the earth, etc. And it is complicated more because the moon moves in an elliptical orbit, not just straight past the sun, etc.

Now it makes sense to me. If I'm wrong, feel free to tell me which of my logic is wrong.

.

On a related topic, how long will totality last, across the U.S.? It will start on the west coast around 10:17am (1:17pm eastern time). It will end on the east coast around 2:48pm (eastern time). So for one and a half hours, the shadow will sweep across the country, from coast to coast.

Within that time period, based on the 3 to 4 hour time difference between the coasts, the earth only rotates about half the same distance.

But actually, the earth and the shadow are moving in the same direction... so if the earth weren't turning, the shadow would traverse the distance even faster.

2017/08/16 Corrected some words above. I was mixing up the words "right" and "left", even though I was visualizing it correctly. I'm used to thinking of the west coast as on the left side and the east coast on the right side. For the above, my perspective is from the center of the county looking south. So the west coast is on the right, not the left.
Although since the sun will be pretty high overhead during the eclipse, "left" and "right" aren't good words to use to begin with.

I bought some eclipse sunglasses on Tuesday. Seems like I did it in the nick of time, as they are becoming scarce around here. I got some cheap cardboard ones, and some slightly less cheap 2x magnification cardboard ones (I ordered the latter from Best Buy in the morning, and picked them up after work. When I checked the website in the evening again, they were sold out.) The glasses are neat! You can look at the midday sun while it's high in the sky! What you see is a nice orange disk. That may not sound special, but it's neat to be able to look straight at the sun without it being sunrise or sunset. I'm not sure I could see any sunspots or flares with these glasses, but they should be fine for watching the eclipse. As long as we are lucky and have clear skies. The forecast isn't looking good so far... a 50 to 60% chance of thunderstorms all day on the 21st till 8pm. But things may change, and even with thunderstorms, there might be a break in the clouds. (please, pretty please, at least during totality, please?)

I've been debating whether to work from home that day, or go in to work like usual and take my lunch break during the total eclipse. Now I think I'll work from home... that way I can occasionally check what is broadcast from the other parts of the country that experience the eclipse before we do.

I replaced the pull-chain light switch for the light fixture on Qiao's ceiling fan with a 3-way pull switch. Now we can turn on either 2 or all 4 lights, where before you could only turn on all 4 at once. Now while sitting on the sofa, we can turn on only the 2 lights which face towards the other direction, so that the room isn't uncomfortably dark, but without as much glare from above as before. When we want more light, we can turn them all on. Previously, I had replaced the lamp shades and bulbs, but it was still too bright for me.

One of the metal spines on my umbrella broke. I can't think of any simple way to fix it. It's a fairly new umbrella which my neighbors gave me recently, as thanks for looking after their dogs while they were on a trip. A nice lime-green color with reflective edging. I can't bear to throw it away. I couldn't even bear to throw away my old umbrella yet.. On it, the fabric had worn out and had holes. I cut the fabric off that one, and still have the metal umbrella skeleton. It's pretty useless, slightly dangerous, but looks neat in a goth-steampunk kind of way.

My dad has been researching his side of the family tree. It's amazing what you can find in old census records. This FamilySearch website is run by the LDS church, and lets you search records for free. There's an 1885 census from the state of Nebraska, with an entry for my grandpa's father and grandfather. We also found 1870 and 1880 census records which seem to match, but there are some discrepancies which we haven't been able to explain yet. We found that the Nebraska Historical Society has microfilms of church records from the area my grandpa's grandpa lived. Those records don't seem to be online anywhere. My dad contacted them, and they have a volunteer who will check the microfilms for us, even translating from Latin! How nice is that?

The path of the total solar eclipse that will happen on August 21 includes Columbia, SC, where I live!

The total eclipse path in SC also includes Greenville and Charleston, two of the other largest cities/towns in the state.

This NASA map shows more details, and the area that will get a 90% eclipse is pretty big. I imagine 90% will be pretty stunning to see, too. But it may not be, because we had a partial eclipse before which was nearly unnoticeable. A small amount of sun is still very bright.

I hope it won't be cloudy here on that day.

At work, we're using Eclipse for our Java development environment.

Something I don't understand, is why Eclipse thinks it needs to recompile my projects when a jar file on the Build Path has changed.

We have jar files which get updated fairly often. The jar files are built in a separate project, and then promoted into one of the projects that I work with. It takes about 20 minutes for Eclipse to rebuild my projects from scratch, so I don't want Eclipse doing a full rebuild whenever one of the jar files is updated. I only want Eclipse to build the Java files that have changed.

Eclipse indicates that it is doing a "scoped incremental build", but in reality, it first cleans the output folders, so that it ends up doing a full rebuild of all the code.

Originally, my build path included the jar files in my workspace. I tried to avoid the problem by copying the jar files to a folder outside of my workspace, and by updating my build path to point to the files in that other folder.

This partially fixed the problem. Eclipse no longer did a full rebuild when I manually selected to build the projects. It only built the changed Java files, as I'd expect. But when I clicked to start debugging the projects, *then* Eclipse did a full rebuild after all.

I found out that there is a setting in the Preferences - "Build (if required) before launching". Presumably unselecting that option would avoid the full rebuild when I start to debug.

But my question is, why does Eclipse think that a rebuild is required, when only a jar file has changed? Is it really necessary? Shouldn't the jar file changes be automatically picked up at run-time, without recompiling the calling code?

The only thing I can think that the rebuild may accomplish, is to flag any errors which may exist due to mismatches between the calling code and the jar files. It shouldn't actually result in any changes to the compiled class files, right? Or am I mistaken?

When I've updated my workspace from the repository, there shouldn't be any mismatches between the Java modules and the jar files, so I don't see any benefit to doing a full rebuild.

I may need to find an Eclipse forum to post my question.